The invention relates to a process for preparing cycloalkenes, in particular cyclohexene, by partially hydrogenating the corresponding aromatic hydrocarbon, in particular benzene, in the gas phase in the presence of water vapor and in the presence of a ruthenium catalyst.
An already known process is described in GB Pat. No. 1,094,911, now laid open to public inspection, the disclosure of which is incorporated herein by reference. According to this known process, the aromatic hydrocarbon (in the vapor phase) being converted is brought into contact with metallic ruthenium, for instance ruthenium-black, together with hydrogen and from 5 to 50% by weight (relative to the hydrocarbon) of an alcohol, at a temperature of 15.degree. C. to 150.degree. C. The ruthenium may be employed on a support.
From an experiment, it appears that it is only possible to prepare cyclohexene from benzene in the gasphase according to this already known process at a conversion of only 10% and a selectivity of only 7%.
This reveals the great disadvantage of this known process: the very low selectivity to the desired alkene while also at a low rate of conversion of the hydrocarbon.
From GB Pat. No. 1,381,048 which has been laid open to public inspection, the disclosure of which is incorporated herein by reference, it is known to hydrogenate benzene to cyclohexene in the presence of water. However, this process is a liquid-phase process in which the water is used together with an alkaline substance to maintain a liquid phase at pH above 7.5.
However, liquid-phase hydrogenation has major drawbacks. Among other things, liquid-phase hydrogenation is not a very safe process, is a much more difficult process to operate continuously, and demands a relatively higher total pressure than gas-phase hydrogenation. Liquid-phase hydrogenation processes also require more complex equipment particularly for separating the reactants from the catalyst suspension.